1. Field of the Invention
The present invention relates to a device and a means for determining the firmness of objects, and more particularly to a device and means for non-destructively measuring the firmness of objects using a fluid jet and a light beam specifically for determining the firmness of food products.
2. Description of Prior Art
Firmness, i.e., resistance to deformation, is a key factor in determining the quality of food products. Consumers consider firmness as a predictor of the storing ability and eating quality of fresh fruits and vegetables. Food buyers use firmness when selecting which lot to purchase. Firmness is also a key factor for growers in deciding on harvest dates and in sorting products at packing houses.
One of the primary functions of a fresh fruit and vegetable packing house is to convert a highly variable incoming flow of a product into packages containing products with uniform quality. Many products continue to ripen after harvest, therefore the items packed must be firmer than desired by the end user. Thus, a critical operation at packing houses is to remove riper items which are often the highest quality and more valuable for regional markets but would become soft and cause wholesale buyers to reject the entire shipment when delivered to distant markets. Soft fruit was also the most frequently reported physiological disorder recorded on USDA inspection forms for plum, peach, and nectarinc shipments to the New York market between 1972 and 1985, where 40% of the shipments inspected were rejected for being too soft, Ceponis, Cappellini, Wells, and Lighther, "Disorders in plum, peach, and nectarinc shipments to the New York market, 1972-1985," Plant Disease 71(10) 947-952 (1987).
Unacceptable variations in firmness frequently occur during food production, manufacturing processes, and product storage. A common approach for minimizing variability and for marketing products with uniform firmness is to separate items into groups with similar firmness levels.
Presently, manual separation is the only practical method available to packing houses for firmness sorting. The sorting task is labor intensive, monotonous, and inaccurate. Consequently, there is a strong need for development of a mechanical device to separate objects based on firmness.
There are several methods of measuring firmness. One method measures firmness by destroying the item under test. With this method, one randomly selects samples from a lot and measures them under the assumption that they represent the total-population. The traditional measure of fruit firmness is with a penetrometer described in Magness and Taylor, "An Improved Type of Pressure Tester for Determination of Fruit Maturity," USDA Circular 350 (1925). This device destructively measures the firmness of an object by determining the force necessary to penetrate the object with a probe to a predetermined depth. A more recent method is disclosed in Studman and Yuwana, "Twist Test for Measuring Fruit Firmness," 23 J. Texture Studies 215-227 (1992). This reference presents a firmness measurement method based on the moment necessary to rotate a blade attached to a spindle after it is pushed into the fruit.
A disadvantage of destructive testing is that to achieve higher levels of reliability one must destroy greater numbers of the product, and one cannot measure the firmness of every item going through a packing line. Furthermore, the ability for a sample to predict the condition of the lot is especially weak for fruit because weather and other variables prevent the control of processes which affect firmness. Thus, a lot will have large variations that are only partially reduced by manual sorting.
Destructive tests for firmness continue, largely because suitable sensors are not available for measuring firmness of all items in a lot. Consequently, effort has been expended on several approaches for finding a non-destructive firmness method. Such methods have either required mechanical contact between the product and a solid probe or measurement of a secondary property which is subsequently correlated to firmness.
A non-destructive mechanical contact measurement is described in Mizrach and Ronen, "Mechanical Thumb Sensor for Fruit and Vegetable Sorting," 35(1) Transactions of the ASAE 247-250 (1992). In this reference, a "mechanical thumb" is used to measure force-deformation. With this method, the product-is deformed by a pin connected to a pivot arm with a micro-switch. Measurement of firmness by deformation using two steel balls pushed against opposite sides of the fruit has been described in Mehlschau, Chen, Claypool, and Fridley, "A Deformeter for Non-Destructive Maturity Detection of Pears," 24(5) Transactions of the ASAE 1368-1375 (1981). Dawson, "Non-destructive Firmness Testing of Kiwifruit," Programme Information and Abstracts of the Second International Symposium of Kiwifruit, Feb. 18, 1991, Massey University, Palmerston North, New Zealand describes a non-destructive firmness tester for kiwifruit based on the displacement of a small mass pushed against the fruit by a spring. A micro-switch indicates when deformation exceeds the present limits. A single lane firmness sorting machine is described in Delwiche, McDonald, and Bowers, "Determination of Peach Firmness by Analysis of Impact Forces," 30(1) Transactions of the ASAE 249-254 (1987). This machine is used for sorting peaches and pears into hard, firm, and soft categories by analyzing the force from the fruit impacting a plate supported by force transducers. The sorting index used in this reference uses a relationship between the peak force and the time required to reach the peak force.
Measurement of firmness by deformation caused by an applied force is possible because the slope of a force-deformation curve is the modulus of elasticity, or stiffness of the product. A standard is available for "Compression test of food material of convex shape," ASAE S368.1, in ASAE Standards, 39th Edition, Am. Soc. Agr. Engrs., St. Joseph, Mich. (1992). Terms are defined and specifications are given for tests using parallel plates, a single plate, a spherical indenter on a curved surface, and a spherical indenter on a flat surface. A section on calculations provides standardized methods for finding force and deformation on bio-yield and to rupture, point of inflection, modulus of deformability, and stress index. An equation relating impact force to time is provided by Zhang and Brusewitz, "Impact force model related to peach firmness," Trans. of the ASAE 34(2) 2094-2098 (1991). This equation relates to the force-time response of peaches impacting a load platform. Other researches have reported on measurements of quantities such as coefficient of restitution and penetrometer peak voltage (maximum deceleration).
Measurement of secondary properties is described in Perry, "A non-destructive firmness (NDF) testing unit for fruit," Trans. of the ASAE 20(4) 727-767 (1977). The device descried in this reference measures firmness using low-pressure air simultaneously applied to small areas on opposite sides of peaches. A rotating steel drum has been used for separating soft oranges from undamaged ones is described in Bryan, Anderson, and Miller, "Mechanically Assisted Grading of Oranges for Processing," 21(6) Transactions of the ASAE 1226-1231, (1978). Finney, "Mechanical Resonance within Red Delicious Apples and its Relation to Fruit Texture," 13(2) Transactions of the ASAE 177-180 (1970) describes non-destructive techniques based on resonant frequencies to evaluate firmness of apples and peaches. The concept of using resonant modes in a sorting machine is described in Peleg, U.S. Pat. No. 4,884,696. (1990). In addition, blueberries and grapes have been sorted by low frequency vibration, Chen and Sun, "A review of non-destructive methods for quality evaluation and sorting of agricultural products," J. Agric. Engng. Res. 49, 85-98 (1991). Nuclear magnetic resonance data from fruit has also been used, Stroshine, Cho, Wai, Krutz, and Baianu, "Magnetic resonance sensing of fruit firmness and ripeness," ASAE technical paper no. 91-6565, ASAE, St. Joseph, Mich. (1991).
Both the non-destructive contact sensors and the sensors that rely upon secondary characteristics share the common problem that they are mechanically complicated and are too slow for packing house operations. A common difficulty is the need for mechanical contact between, the product and the sensor. Mechanical devices limit the speed of operation (except for vibration) and can limit reliability. Speed is also limited in some of the approaches by the need to completely analyze the signal generated. Secondary methods, such as measuring resonant frequencies to evaluate firmness (which may involve striking the fruit with a hard object) often requires that the impulse location be held constant for repeatability. Moreover, there is a need to correlate the results obtained with force-deformation relationships of the fruit which are important to end users and other buyers.
Several non-contact firmness sensors have been developed in testing human eyes for glaucoma. For example, the tonometers of Stauffer (U.S. Pat. No. 3,181,351) and the apparatus of Motchenbacher (U.S. Pat. No. 3,232,099) deform the eye with puffs of air, illuminate the point of deformation with noncoherent light, and then determine the amount of deformation based on the intensity of the light reflected off of the point of deformation. However, these devices are incapable of measuring the firmness of objects with rough surfaces (e.g. fruits) because the intensity of the light reflected off of the surface is a function of both the amount of deformation and the roughness of the reflecting surface--a highly variable parameter in fruits and many other products. Moreover, tonometers designed for testing human eyes are not suitable for testing objects with widely varying ranges of firmness, e.g., tomatoes and apples; furthermore, they are adapted to test objects of rather uniform size, convexity, and firmness and must be carefully aimed at a specific point (the front of the eyeball) to obtain valid measurements.
There is thus a need for a method and apparatus of testing entire lots of fruit effectively, efficiently and non-destructively. There is further a need to directly measure firmness rather than-secondary properties, and to do so without mechanically contacting the fruit so as,to avoid possible damage thereto. It would also be desirable to have a method and apparatus for testing the; firmness of objects having variable surface properties, such as rough surfaces with variable reflectance.